1. Technical Field
The present invention pertains to the art of permanent magnet (PM) AC synchronous machines, and more particularly, to a system and method for determining rotor shaft position of a high voltage machine using auxiliary windings.
2. Background Art
There are a variety of known PM AC synchronous machines or motors on the market. One example of a known PM AC synchronous machine is depicted in FIG. 1. More specifically, FIG. 1 shows machine winding details as seen from an air gap, looking toward stator teeth 10 and winding slots, of a particular three phase prior art PM machine structure. The machine depicted is a 48 pole machine with two similar sets of three concentrated phase windings per pole pair. This type of machine construction is referred to as a ½ slot per pole per phase fractional slot winding machine. Thus the 48 pole machine has 72 stator teeth/slots. The windings 12 are said to be concentrated because each winding coil 12 is wrapped around a single tooth 10. There are two sets of 3-phase windings allowing for two 3-phase power electronic drives to power/control the machine. All phase A coil windings 12 in either winding set are connected in series such that the voltages induced by the rotor magnets in the phase A coils all “add” algebraically. The connections for the phase B and phase C coil windings are similar, all in series connection. For this particular machine the individual coils of the main or “power” windings, those with their external terminals connected to the power electronic drive, are wound with Nt=180 turns per coil 12. Each phase of either winding set then has twelve (12) series connected coils, for a total series number of turns of 2160 turns per phase. This high value of turns per phase is due to the fact that this particular machine is intended for high voltage operation, with thousands of volts between the phase winding sets. Note that the designations of α and β in FIG. 1 are used for identification purposes only, and these designations are not to be confused with reference to the use of Greek characters in the mathematical relationships described in the method of the present invention. FIG. 2 depicts a partial cross sectional view of the prior art machine of FIG. 1. More specifically, FIG. 2 shows stator teeth 10 extending from a stator lamination stack 14.
Standard operation of a PM AC synchronous machine requires that the machine stator winding currents must be time and spatially synchronized (in-phase) with the rotating position of the multi-pole magnets in the rotor structure. In essence, the multi-pole magnetic fields in the machine air gap due to the currents in the stator phase windings must rotate synchronously with like multi-pole magnetic fields in the air gap due to the rotor magnets. Therefore to drive or control the stator currents to obtain synchronous operation one must be able to determine instantaneous rotor position information. There are many ways to obtain rotor position information in a PM AC synchronous machine, but the most rugged and desirable measuring systems to obtain this information, particularly in harsh environments, do not utilize mechanical or optical measuring subsystems, rather, only simple voltage and current measuring instruments or transducers are employed. Such measurement and position sensing systems are commonly termed “sensorless” systems, even though they do utilize voltage and current “sensors.”
It is well known in the electrical practice (see for example the textbook of Peter Vas, “Sensorless Vector and Direct Torque Control,” Oxford University Press, 1998, particularly Section 3.1.3.2, pages 124 through 136) that the shaft position (the instantaneous rotor angle) of a multiphase PM AC synchronous machine can be determined by:                1) measuring the machine stator terminal voltages;        2) subtracting off winding resistive voltage components;        3) integrating the resultant voltages to obtain winding flux linkages;        4) forming equivalent two-phase winding flux linkages that are in time quadrature to each other; and        5) subtracting off equivalent two-phase winding inductive flux linkage components due to the equivalent two-phase stator winding currents.The resultant set of two flux linkages, after operations 1) through 5), are in time quadrature to each other (sine and cosine function like time variation) and, more importantly, are the equivalent two-phase stator winding flux linkages due solely to the rotating array of rotor mounted magnets. Therefore the time variations of these two flux linkages, here referred to as psim_alpha and psim_beta, which vary in sine and cosine function fashion, as functions of the instantaneous rotor angular position, directly indicate the angular position of the set of rotor magnets. That is, if theta m is the mechanical angular measure of the rotor with respect to a fixed point on the stator air gap surface, the variation of flux linkages psim_alpha and psim_beta will be in proportion to the functions sin(np*(theta_m+theta_mo)/2) and cos(np*(theta_m+theta_mo)/2), where np is the number of magnetic poles of the rotor structure and theta_o is a fixed angle offset determined by the reference point for theta_m on the stator air gap surface. One can determine theta_mo by inspection from the placement of the particular stator winding phase coils in the stator structure. For example it is common to assign the rotor mechanical position theta_m reference (value=0.0) as the angle at which the flux linkage due to the rotor magnets is maximum and positive in the a-phase set of stator coils. The flux linkage fundamental sinusoidal function component, due to the rotor magnets, then varies for the a-phase set of rotor windings as psim_a=M*cos(np*theta_m/2), where M is the peak magnitude of the coupled flux linkages (measured in Weber-turns). The flux linkages due to the rotor magnets in the other phase windings of the machine then follow in phase sequence, separated in angular variation by the angle np*delta_theta/2, where delta_theta is the mechanical angle between adjacent phase windings in the stator structure. If one can then extract the angular variation of the flux linkages due to the rotor magnets, then the angular variation of the rotor itself is then determined. This extraction process is best done not on the phase flux linkages themselves, but rather on the calculated equivalent two-phase flux linkages psim_alpha and psim_beta, which can take advantage of their true phase quadrature nature and a simple phase locked loop system which filters out measurement noise (from measurements of the phase winding currents and terminal voltages). As mentioned, the basic theory and practice of this method, or close variations of this method, for rotor angle estimation are well known in the art.        
The rotor angle estimation system described above is hard, or at best expensive, to implement for very high voltage machines. Direct measurements of magnitudes and phases of very high voltages (thousands of volts) at machine terminals are not trivial. Even tapped coil voltage measurements are not trivial due to very high common mode voltages with reference to low voltage signal processing electronics. But it is still desirable to be able to utilize the rotor position estimation scheme described above even for very high voltage machines. Therefore, there is seen to be a need in the art for new systems and methods to indirectly determine the required high voltage coil flux linkages